1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory having a stacked gate structure including a floating gate, and to a method of fabrication thereof, and in particular, the present invention relates to an improved technique for reducing poor fabrication of such a nonvolatile semiconductor memory.
2. Description of the Related Art
A conventional nonvolatile semiconductor memory such as an EEPROM or the like (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 7-249746) has a memory array in which the word lines and bit lines intersect in a plane, and transistors having stacked gate structures, which are formed from a floating gate and a control gate, are formed at these intersecting portions.
In such a nonvolatile semiconductor memory, the control gate is formed of an electrically-conductive film of polysilicon or the like. In order to make this polysilicon be conductive with the circuits at the periphery of the memory array (the peripheral circuits) and form the word lines, usually, contacts must be opened, and the polysilicon and the metal wires (e.g., aluminum wires) at the peripheral circuits must be connected via these contacts.
However, in recent years, accompanying the advance of microfabrication technology in semiconductor memories, the intervals between adjacent word lines (the polysilicon) have become more narrow. For this reason, at the time of opening the aforementioned contacts, it has become difficult to open the contacts with precision above the polysilicon where the control gates are formed. If the contacts cannot be opened with precision above the polysilicon, there is the concern that problems will arise, such as the metal wires at the peripheral circuits will be conductive with the substrate of the nonvolatile semiconductor memory, or the like. Namely, in the process of opening the contacts, over-etching is carried out in order to avoid poor opening. However, there is the concern that, due to this over-etching, the field oxide film will be etched and the contacts will reach to the substrate. If the contacts reach to the substrate, the problem arises that the electric signals from the peripheral circuits cannot be transferred to the control gates via the word lines.
On the other hand, due to the capabilities of the fabricating devices and the like, there are limits on precisely opening contacts above the polysilicon regardless of the intervals between the adjacent word lines (the polysilicon).
From the above, there have been desired a nonvolatile semiconductor memory of a structure in which electric signals from peripheral circuits are reliably transferred to control gates via word lines even if contact holes cannot be precisely opened above the word lines, and a method of fabricating such a nonvolatile semiconductor memory.